The Bcl-2 family proteins are important regulators of the intrinsic apoptosis pathway. The Bcl-2 oncogene was first identified in follicular lymphoma where the t(14;18) chromosomal translocation results in significant overexpression of the protein in B cells. In contrast to other known oncogenes, Bcl-2 does not stimulate cellular proliferation, but rather inhibits programmed cell death by protecting cells from a wide variety of pro-apoptotic stimuli, including cytokine withdrawal, irradiation, cytotoxic drugs, heat and deregulated oncogenes. The Bcl-2 family of genes encodes a family of closely related proteins that possess either pro-apoptotic or anti-apoptotic activity and share up to four Bcl-2 Homology (BH) domains. The anti-apoptotic family members (Bcl-XL, Bcl-2, Bcl-w, Bcl-B, A1 and Mcl 1) are characterized by four BH domains that are designated BH1-4. The pro-apoptotic family members can be further subdivided into multidomain proteins (Bax, Bak) and the BH3-only proteins (Bad, Bik, Bid, Bim Hrk, Bmf, Noxa, Puma). The interplay between these three groups of proteins serves as the gateway to the intrinsic apoptosis pathway. ABT-263 is a novel small molecule Bcl-2 family protein inhibitor that binds with high affinity (Ki is less than or equal to 1 nM) to multiple anti-apoptotic Bcl-2 family proteins including Bcl-XL, Bcl-2, Bcl-w, and Bcl-B. ABT-263 displays potent mechanism-based cytotoxicity (EC50 is less than or equal to 1 microM) against human tumor cell lines derived from small cell lung carcinomas and lymphoid malignancies. ABT-263 exhibits potent single agent activity against 10 of 22 cell lines consisting of multiple leukemia and lymphoma types spanning both B-cell and T-cell malignancies. In two flank models of diffuse large B-cell lymphoma (DoHH-2 and WSU-DLCL2), significant monotherapy activity was noted when ABT-263 was administered at a dose of 100 mg/kg/day given by mouth, once a day for 17 days. Both of these tumors are known to express high levels of Bcl-2 due to the t(14;18) translocation. The WSU-DLCL2 line was isolated from a patient whose disease progressed following chemotherapy, radiation therapy and bone marrow transplantation, and is recognized as a model of therapy-resistant lymphoma.